The present invention relates to a method for processing phenolic and epoxy resin prepreg honeycomb sandwich panels to achieve a smoother, more paint-ready surface post-production. Prepreg is a common term for fabric reinforcement that has been pre-impregnated with a resin system. The resin system is typically an epoxy that already includes the proper curing agent. As a result, the fabric is ready to lay into a mold without the addition of additional resin and without the steps required of a typical hand lay-up. These pre-impregnated composite fabrics and tapes, or “prepregs,” are used in a large number of applications. For example, composite sandwich panels formed of reinforcing fibers and a matrix resin are widely used for aircraft, automobiles and other industrial applications because of their strength and weight characteristics. The details of the process by which these panels can be formed is described generally in U.S. Pat. No. 7,186,310 entitled Method For Forming A Honeycomb Composite Sandwich Panel, the contents of which are fully incorporated herein by reference. Advances in the science of sandwich panels have led to greater mechanical properties and the continuing search for more efficient manufacturing techniques. One constant goal is to limit and decrease the amount of surface defects, such as pocks, voids, and creases, which lead to both mechanical defects as well as costly post-production operations.
Especially for structural materials and interior materials of aircraft, reinforced fiber composite materials are increasingly used as skin panels in honeycomb sandwich panels for reduction of weight. Honeycomb cores used include aramid honeycomb cores and aluminum honeycomb cores. It is generally practiced to produce a honeycomb sandwich panel by laminating a honeycomb core made of aramid paper with prepreg laminates on both sides and curing the prepreg laminates while bonding the prepreg laminates to the honeycomb core as so-called co-curing. In this case, the adhesive strength between the honeycomb core and the prepreg laminates as skin panels is important, and a method of keeping adhesive films between the honeycomb core and the prepreg laminates and curing the prepreg laminates together with the adhesive films for fabricating a sandwich panel has been popularly used.
Furthermore, to decrease surface defects such as pits and resin blurs on the panel's skins, it has been often practiced to stick adhesive films on the surfaces of prepreg laminates, to allow them to cure together with the prepreg laminates. However, to advance weight reduction, and reduce material and fabrication costs, it would be desirable to form smooth skin panels free from surface defects without using any adhesive film.
In typical manufacturing operations for aerospace applications, the panels are manufactured and then there is a fill and fare operation prior to painting the panel where all of the defects are addressed. That is, the manufacturing process produces a panel surface that contains inherent and unacceptable defects such as pitting and telegraphing. Pitting leaves very small pinholes in the surface of a panel that must be filled prior to any painting. Telegraphing is a result of the facing material drooping into the empty area of the honeycomb core. This produces a surface that is not perfectly flat and can be observed after the painting process unless further remediation is performed on the finished panel. To eliminate these defects, the “fill and fare” operation is undertaken where the defects are cured.
The first remedial operation that typically occurs is a fill and sand step, where putty material is spread onto the surface to fill in the gaps, pits, and holes. There is significant labor in this step, which adds costs to the panel, and adds a slight weight increase as well due to the added weight of the putty material. Alternatively, surface films can be added to provide smooth surface to the panel. However, there is increased material costs associated with surfacing films that add weight to the panel as well as labor costs needed to apply the film. In some cases, “peel ply” materials are used, which also add weight and time to the manufacturing process.
All three of these remedial steps use expendable release materials to process the panels. These fill and fare operations are both time consuming and costly, and adds significantly to the manufacturing costs of the panels. It would be desirable to eliminate or significantly reduce the amount of fill and fare procedures prior to painting. The present invention addresses this need. By altering the choice of release materials and processing parameters to achieve the desired smooth surface, the present invention eliminates or reduces the amount of extra labor and material costs of the panel than would otherwise exist. The present invention adds the significant benefit of also adding no additional weight to the panel.